Synthetic aperture radar can be used to generate high resolution images of a target. The target is illuminated by pulses of radiation and the echoes of the pulses are detected and processed to form image data. SAR can be used on a spacecraft, such as a satellite. The raw echo data in digital form is typically referred to as a Level 0 product. SAR images formed from the Level 0 products are typically referred to as a Level 1 product.
Typically, the Level 0 raw data obtained by SAR on satellites is processed on ground into a Level 1 product and the satellites therefore have to store the Level 0 data until it can be transmitted to ground. This can be a problem when the satellite does not frequently have the opportunity to communicate with a ground station. For example, on planetary missions, the satellites would have to store received data for an extended time. Moreover, satellites such as low Earth orbit (LEO) satellites would have to store data when they are not in communication with a ground station. The number of images that can be obtained is then limited by the available storage on the satellite.
It is sometimes desired to have more than one SAR antenna on a satellite in order to, for example, generate interferometric images or complex coherence functions. By increasing the number of SAR antennas, the amount of Level 0 raw data generated is also increased, exacerbating the problem.
It has been proposed that there may be advantages to carrying out image processing on the satellite. For example, if lower resolution images are acceptable, the level 0 data could be processed into lower resolution images on the satellite and thereby allow the satellites to store data for a larger number of images than if the stored data was Level 0 data. Moreover, in some applications it is desired to provide real-time or near real-time monitoring of a target scene and it may take too long time to send the image data to ground for processing.
State of the art SAR image processing is implemented in software on ground. Currently available CPUs suitable for space on-board processing would be too slow to run the computationally intensive software used on ground effectively. This is because the processing rate of fault tolerant general purpose CPUs suitable for space on-board processing is orders of magnitude less than terrestrial systems.
The invention aims to address the above and other problems.